Cost-Effective Reagents as Defoamers and Crystal Modifiers to Enhance the Filtration of Phosphogypsum

Phosphoric acid is an important intermediate product for production of fertilizers. It is mainly produced by the wet process, in which phosphate concentrate is leached with sulfuric and weak phosphoric acids to produce phosphoric acid. Calcium sulfate crystallization occurs as leaching is taking place. According to the process adopted, calcium sulfate dihydrate (gypsum) (CaSO4.2H2O) or calcium sulfate hemihydrate (CaSO4.0.5H2O) is crystallized. Filtration of gypsum to recover phosphoric acid is the bottleneck in this process. Therefore, the major goal of this project is to enhance filtration of phosphogypsum using cost-effective reagents. Clarification of the mechanisms controlling the effect of such additives is another important objective in this study. In order to achieve these goals, several tasks have been conducted including: (1) bench- scale testing of effect of two different surfactants (Crysmod and HiFlo-S5*) on crystal modification and filtration of phosphogypsum produced from South Florida high magnesium phosphate concentrate, at low, medium, and high sulfate levels, (2) role of techniques and point of addition of surfactants, (3) cost-benefit analyses, and (4) basic studies to elucidate the mechanisms controlling the effect of surfactants under different conditions.

The results suggest the following:

Optimum filtration results could be obtained by addition of surfactant as a mixture with water during nucleation.

Both surfactants can improve filtration rate at all sulfate levels.

At any sulfate content, both P2O5 recovery and reaction efficiency are higher with the addition of surfactants.

Both surfactants decrease the induction time at different super-saturations. Also, larger pure calcium sulfate dihydrate crystals with higher mean crystal diameter are produced in the presence of surfactants.

A gain of about $1.0 / ton P2O5, could be realized if Crysmod is used. However, the gain will be about $4.0 / ton P2O5 if HiFlo-S5 is used. Such gain is in addition to the increase in filtration rates due to use of these surfactants.